This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP99/00441 which has an International filing date of Feb. 3, 1999, which designated the United States of America.
The present invention relates to a polarizing element for taking only unidirectional plane-polarized light out of natural light being so called indefinitely polarized light.
A conventional liquid crystal projector (LC projection display) is provided with a liquid crystal display panel that necessarily uses a polarizing plate for cutting off unnecessary components from light outgoing therefrom for forming an image. However, a conventional dichroic type polarizing plate absorbs unnecessary light that becomes heat energy for internally heating the plate, causing premature deterioration of the plate. Accordingly, recent LC projectors use, instead of the polarizing plate, a polarized-beam splitter having a thin-film coat deposited thereon or diffraction type polarizing elements disclosed in Japanese Laid-Open Patent Publication No. 61-240204 and No. 62-249107 and Japanese Patent No. 2594548 (Japanese Laid-Open Patent Publication No. 63-26604).
However, the above described conventional methods have the following drawbacks:
The thin-film deposited type polarized-beam splitter is constructed of two glass-made prisms bonded to each other, which is thick and, therefore, is not so small and light to be usable in liquid crystal projectors. Namely, the use of this type beam splitter as a polarizing plate of a light outputting portion of a liquid crystal projector necessarily elongates a back focal length of a projector lens and, thereby, causes the need for increasing a diameter of the projection lens, a projection distance and a weight of a whole optical system.
The application of the diffraction type polarizing element described in Japanese Laid-Open Patent Publication No. 61-240204 as an emergent-side polarizing plate cannot produce a well-contrasted clear image since the element cannot completely split polarized light, causing leakage of light. The diffraction type polarizing element described in Japanese Patent No. 2594548 (Japanese Laid-Open Patent Publication No. 63-26604) as compared with the element of Japanese Laid-Open Patent Publication No. 61-240204 has a higher degree of polarized-beam separation, a higher contrast and a smaller wavelength-dependent diffraction difference that is estimated at a value of dispersion of refracting media. This element, however, cannot have a large diffraction angle because of a small differential refractive index of its diffraction grating. The use of this element as a polarizing plate on the emergent side of a liquid crystal projector elongates a back focal length of a projection lens. The hologram-type polarized-light splitting element described in Japanese Laid-Open Patent Publication No. 62-249107 has an excellent polarized-light splitting ability and a large splitting angle but has an incident light falling thereto at a large angle to a line perpendicular to the hologram plane, requiring a large working space. Therefore, the use of this element as an emergent-side polarizing plate in a liquid crystal projector may also require elongation of a back focal distance of a projection lens.
In view of the foregoing, the present invention was made to provide a hologram polarized-light splitting element capable of effectively splitting polarized light with no absorption of unnecessary light, i.e., eliminating the possibility of being heated by absorbed light and of deterioration by heat, which element is so light and small to form a compact optical system. The present invention is also directed to a compact liquid crystal display device using the above-described element.
An object of the present invention is to provide a hologram-type polarized-light splitting element made of a glass substrate with a hologram formed thereon, which element can split incident light into two plane-polarized (linearly polarized) components having polarization directions perpendicular to each other by allowing one of the components to pass the hologram after diffraction and the other component to pass straight without diffraction, and which element can select and emit only the non-diffracted plane-polarized component of the light in the specified direction and, at the same time, can further reflect the diffracted plane-polarized component at least once at a boundary between the glass substrate and medium surrounding the element. This element can thus split incident light at high accuracy with no absorption of unnecessary light component that may become thermal energy as observed in the conventional dichroic absorption type light polarizing element, thereby it can maintain high reliability of performance for its long service life. The element is a thin plate that can form a very compact and light optical system in which it can be disposed at right angles to an optical axis of the system.
Another object of the present invention is to provide a hologram-type polarized-light splitting element made of a glass substrate with a hologram formed thereon, in which the hologram is given diffraction properties for obtaining diffracted plane-polarized light and non-diffracted plane-polarized light from a p-plane-polarized component and a s-plane-polarized component, respectively, of incident light at a high separation degree by diffracting the former component to travel in the direction making a separation angle of 60 degrees with the travelling direction of the latter.
Still another object of the present invention is to provide a hologram-type polarized-light splitting element made of a glass substrate with a hologram formed thereon, in which the hologram is given diffraction properties for obtaining diffracted plane-polarized light and non-diffracted plane-polarized light from a p-plane-polarized component and a s-plane-polarized component, respectively, of incident light at a high separation degree by diffracting the former component to travel in the direction making a separation angle of 48.2 degrees with the travelling direction of the latter.
A further object of the present invention is to provide a liquid crystal projection display having a compact optical system with a short back-focal distance of a projection lens, which comprises a liquid crystal panel and at least a first polarizing element and a second polarizing element on the incident side and the emergent side, respectively, of the liquid crystal panel, in which each polarizing element is a hologram-type polarized-light splitting element according to the present invention, which element can split incident light into two polarized components at high contrast with no fear of being heated by light absorption and has no need for elongating a back focal length of its projecting lens.